In recent years, a radiation-curable resin, for example, a UV-curable resin, has been used in various fields and for various applications. Such types of resins are only cured at a location therein, which location is irradiated by at least a certain amount of radiation. On the other hand, radiation, for example, UV light, attenuates in the process of traveling into a resin. Therefore, it is difficult for radiation to reach deeper levels. In addition, radiation would be absorbed by a substance absorbent at the same wavelength as that of the radiation. Thus, radiation is greatly subject to attenuation and absorption.
Therefore, a photocurable resin is cured only to a depth of several micrometers to several millimeters. That is, the resin is not cured at deeper levels. Therefore, it is difficult or impossible to apply such resin as a thick material. In addition, if such resin contains a filler, etc., having a characteristic of hindering the transmission of radiation, the curing process would be easily disrupted or even blocked. Therefore, the range of applications has been centered on the fields of photoresists, coatings, paints, adhesives, varnishes, etc. These are the problems of such resins.
In order to solve the above problems, a range of products, represented by the following, has been provided:
An easily curable UV-curable resin (that is, an “active radiation curable composition” from Mitsubishi Rayon Co. Ltd. (see Patent Document 1 (JP 8-283388A))); and
A UV-ray-heat curable resin (that is, “Optomer KS series” from Asahi Denka Co. Ltd.; “Radecure” from Hitachi Kasei Kogyo Co. Ltd.; and “UE resin” from Toyo Boseki Co. Ltd. (see Patent Document 2 (JP 61-38023A), etc.)).
However, an easily curable UV-curable resin is subject to interruption of the curing process when radiation is blocked by a filler, etc. This is a problem that is yet to be solved. A UV-ray-heat curable resin is first irradiated by UV rays and is then heated. The curability by radiation of such a resin is merely as high as that of photocurable resin. That is, the problems with curing of a thick material or with curing of a resin containing a filler are not at all solved. These problems are merely addressed by the thermal curing process to be carried out after the photocuring process (this process is able to cure only a surface layer). That is, these problems have hitherto not been substantially solved.
If a technique is established to quickly cure a thick resin material that contains a radiation blocking substance and has a characteristic of attenuating and absorbing radiation to a large degree, this enables such resin material to be applied not only to the conventional fields of applications but also to various other fields to which such resin material has hitherto been not applicable due to the above problems with photocurable resin. In particular, one such field is that of FRP resins, in particular, CFRP resins.
Conventionally, an FRP lends itself to various processing methods or various manufacturing methods. However, a matrix resin is a thermal curable resin or a thermoplastic resin in most cases. Molding an FRP, in particular, a CFRP, has the following problems, among others. One is that temperature control is complicated and therefore prolongs curing time, resulting in high-cost processing. Another is that curing of a large FRP material requires a large heating furnace. Yet another is that a resin that is curable within a small amount of time at a normal temperature cannot be used for a large FRP material that requires prolonged molding time. Yet another is that change in resin viscosity due to change in temperature changes the stage of resin impregnation, which makes it difficult to carry out the molding process. Yet another is that the residual solvent causes generation of voids during the process of resin curing, resulting in deteriorated quality of the resultant molded product.
Recently, as a solution to the above problems, application of a photocurable resin to a matrix has attracted attention. Such a method of curing a matrix resin may be represented, in particular, by a filament winding method from Loctite Corp., the method using a UV curing process and a thermal curing process together (“fiber/resin compositions and a method of preparing same” from Loctite Corp. (see Patent Document 3 (JP 7-507836A)). However, an FRP molding method using such a composition is problematically executed as follows. First, an FRP is impregnated with resin but is not yet cured. Subsequently, the FRP is irradiated with UV radiation. This causes the surface thereof to be cured. This also causes the interior thereof to be gelled to a large degree. This makes it possible to maintain the shape thereof and the impregnation state thereof to a certain degree. Finally, the curing process is completed by heating.
In this method, very little change in resin viscosity is admittedly caused by change in temperature. In addition, the handling operation after the impregnation process is easy to perform. However, the thermal curing process is necessary for a complete curing. This increases the fuel and light expenses necessary for the thermal curing. This also requires prolonged working time. These, along with other factors, contribute to increases in costs of processing. In addition, completion of the curing process requires a prolonged time. In addition, a large FRP material requires a large heating furnace. These problems, among others, are yet to be solved.
In view of the disadvantages of the conventional radiation-curable resin and of an FRP, in particular, a CFRP, the present inventors have studied a technique to cure a thick resin material, containing an radiation blocking substance, by irradiating it, and a technique to radiation cure an FRP, in particular, a CFRP. As a result, the present inventors have developed a novel technique regarding a resin composition of the chain curing type. This technique involves a novel resin curing method able to also radiation-cure a substance having a characteristic of blocking radiation to a relatively large degree. Such a substance is, for example, a carbon, a carbon fiber (CF), a metal, a resin containing an inorganic filler, etc., (such as carbon fiber reinforced plastics (CFRP), a carbon/metal/inorganic substance containing resin, etc.). This technique also involves a composition used by the method, a molded article produced by the method, and a molding method based on the method. See Patent Document 4 (JP 11-193322A) and Patent Document-4-5 (JP 2001-89639A).
The following is the list of the patent documents:    Patent Document 1: JP 8-283388A    Patent Document 2: JP 61-38023A    Patent Document 3: JP 7-507836A    Patent Document 4: JP 11-193322A    Patent Document 5: JP 2001-89639A